This invention relates in general to solid state analog subscriber line circuits used in telecommunication systems and more particularly to a solid state analog subscriber line circuit having an improved AC impedance matching capability and an improved loop current supply circuit.
Traditionally subscriber line circuits have used inductive type battery feed circuits. These circuits provide excellent performance and low cost but their characteristic components require large amounts of space and have great weight.
Analog line circuits which use lighter and smaller solid state components are finding increased usage in the telecommunications industry today. However, these circuits do not provide performance equal to the inductive line circuits of the past. One of the great disadvantages to the presently known solid state analog line circuits is their inability to function in the presence of high longitudinal voltages and currents on the tip and ring leads of the line circuit.
One technique presently known for overcoming the above mentioned shortcomings is the use of a small transformer and associated circuitry which cancels any magnetic flux in the transformer produced by the loop current. This technique finds disadvantage in that it provides inferior performance relative to a larger transformer and it consumes additional power in the flux cancellation winding of the transformer.
Subscriber line circuits must operate in long loop conditions as efficiently as they do in short loops. In order to supply sufficient loop current to a subscriber instrument on long loops it becomes advantages to use the lowest practical value of battery feed resistance. However, it is also desirable to keep short loop current within some reasonable value to allow the line circuit to operate at a reasonable power level.
It therefor becomes an object of the present invention to provide a solid state analog line circuit which exhibits performance superior to an inductive line circuit in the area of AC impedance matching and loop performance.